Plasma processes are used extensively in the manufacture of semiconductor devices and silicon-based micro-circuits. They are also used in other non-semiconductor applications, such as waveguide and optical device fabrication and in many non silicon-based devices (devices based on III-V materials such as GaAs).
Plasma processing falls into two general categories, namely etching and deposition. In the technique of plasma etching, a substrate is exposed to a reactive gas plasma and material is removed from the surface by the formation of a volatile by-product. By patterning the substrate with a non-erodable mask, a pattern can be effectively transferred into the substrate surface layer. In plasma deposition (Plasma Enhanced Chemical Vapor Deposition (PECVD)), gaseous precursors are introduced into a low pressure plasma where reaction occurs to produce a solid by product which deposits on the substrate as a thin film. For example, SiH4 and N2O are commonly used to produce a SiO2 film.
For both techniques, it is important to terminate the process at an appropriate time, or “end point.” For an etching process this may be the point at which a particular layer has been removed, while for a deposition process it may be the point at which a desired film thickness has been deposited. A number of end point methods based on optical techniques have been described.
Optical Emission Spectroscopy (OES) relies on monitoring the radiation emitted by the plasma and correlating changes in the emission at particular wavelengths with changes in the plasma. Such changes occur as a film is etched and removed since this produces a variation in the plasma composition. This technique is applicable to detecting etch end point, but since no such change occurs as a film is deposited, it is not useful for determining the film thickness in a deposition process.
Interferometry can be used to measure the thickness of a film by measuring the change in magnitude of reflected light that occurs due to interference of light reflected from the top and bottom surfaces of the film. As the film thickness changes (either decreasing in the case of an etching process, or increasing in a deposition process) the intensity of the reflected light varies in a cyclical manner, with the thickness d, corresponding to 1 cycle being given by:d=λ/2n where:λ is the wavelength of the reflected light; andn is the refractive index of the film at the specified wavelength.
The source of the light which is reflected is commonly provided externally (such as a laser or a continuum source). However, the plasma itself can act as the source, in which case the technique is referred to as Optical Emission Interferometry (OEI). In principal the technique will work for both etch and deposition processes.
Common to all optical techniques is the need to provide an optically transparent and vacuum tight window which is necessary to monitor the emitted or reflected radiation. For OEI, the window is ideally located such that plasma emission reflected normally from the substrate surface is viewed. However, to be effective, the presence of this window should not locally perturb the plasma, since this will affect the area of the substrate which is being monitored. This is particularly true in a parallel plate PECVD system where a close electrode spacing is employed to maintain good film quality. Further, for the technique to function correctly over long periods of time, the window must remain optically clear in the presence of reactive plasma and process by-products.
The use of OEI as an endpoint technique in an etching process has been described by Curtis (U.S. Pat. No. 4,328,068). The light pipe used to collect the plasma emission intrudes into, and hence disturbs, the plasma. Also, no provision is made to prevent degradation of the optical components over time due to the etch process.
Likewise Auda et al. (U.S. Pat. No. 5,223,914) describe using a spectrometer in an interferometric mode to measure film thickness during an etch process. The plasma is viewed through a quartz window which has no provision for protection from the plasma environment. Neither Curtis or Auda et al. consider monitoring a deposition process.
Sawin et al. (U.S. Pat. No. 5,450,205) use OEI to monitor multiple points across the surface of a processed wafer using a charge-coupled device (CCD) detector array. This necessitates a large (50 mm) window, which is acceptable for the etch applications discussed, but is unacceptable for a parallel plate PECVD application. The plasma is viewed through a window which has no provision for protection from the plasma environment.
Pirkle et al. (U.S. Pat. No. 5,846,373) describe the use of OEI to measure film thickness in a deposition process. The plasma is viewed through a window mounted in the chamber wall, but there is no provision to protect the window from deposition.
Chen et al. (U.S. Pat. No. 6,071,375) discuss protecting a wall mounted window by means of a purge gas flow through a pre-chamber located between the plasma and the window. Chen et al. do not teach placing the window within an electrode and the configuration is not applicable to a parallel plate PECVD system.
Ookawa et al. (U.S. Pat. No. 6,758,941) describe a window located in a showerhead gas distribution electrode. The window is protected from the plasma environment by means of high aspect ratio apertures located in the electrode. In a close electrode spaced configuration, such as parallel plate PECVD, such features will locally disturb the plasma.
What is needed is a means of mounting a window into a parallel plate PECVD system such that the window does not disturb the plasma and which allows plasma emission to be viewed normal to the wafer, permitting film thickness measurement using OEI.
Therefore, there is a need for improving the optimization of process state functions of a plasma etch process.
Nothing in the prior art provides the benefits attendant with the present invention.
Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement of the semiconductor processing art.
Another object of the present invention is to provide a plasma apparatus for processing a substrate comprising: a vacuum chamber; at least one power supply for generating the plasma in said vacuum chamber; a substrate pedestal for supporting the substrate; an upper electrode assembly having a gas distribution system having a plurality of standard showerhead holes; a detector in optical communication with at least one of said standard showerhead holes, said detector measuring the plasma emission transmitted through said standard showerhead holes; a control system in electrical communication with said detector and said power supply; and optical components positioned in said upper electrode assembly for viewing the plasma emission transmitted through said showerhead holes.
Yet another object of the present invention is to provide a method for monitoring plasma processing of a substrate, the method comprising the steps of: positioning the substrate on a substrate pedestal within a vacuum chamber; introducing a gas through a plurality of standard showerhead holes of a gas distribution system of an upper electrode assembly; generating a plasma from said gas within said vacuum chamber; monitoring said plasma during plasma processing of the substrate, said monitoring occurring by collecting and measuring the plasma emission transmitted through at least one standard showerhead hole, using optical components positioned within said gas distribution system of said upper electrode assembly; and terminating said plasma based on said monitoring step.
The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.